General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in conventional NMR operation the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by .omega..sub.0 =.gamma.B.sub.0, where B.sub.0 is the strength of the static field and .gamma. is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. For hydrogen nuclei, .gamma./2.pi.=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation, T2, called the spin-spin relaxation time. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The net precessing magnetization decays with a time constant T2 because the individual spins rotate at different rates and lose their common phase. At the molecular level, dephasing is caused by random motions of the spins. The magnetic fields of neighboring spins and nearby paramagnetic centers appear as randomly fluctuating magnetic fields to the spins in random motion. In an inhomogeneous field, spins at different locations precess at different rates. Therefore, in addition to the molecular spin--spin relaxation of fluids, spatial inhomogeneities of the applied field also cause dephasing. Spatial inhomogeneities in the field can be due to microscopic inhomogeneities in the magnetic susceptibility of rock grains or due to the macroscopic features of the magnet.
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then a one hundred eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to reverse direction and to refocus. By repeatedly reversing the spins using one hundred eighty degree pulses, a series of "spin echoes" appear, and the train of echoes is measured and processed.
Further background, set forth in the referenced copending parent application Ser. No. 08/936,892, is summarized as follows: The static field may be naturally generated, as is the case for the earth's magnetic field B.sub.E. The NML.TM. nuclear logging tool of Schlumberger measures the free precession of proton nuclear magnetic moments in the earth's magnetic field. See, for example, U.S. Pat. No. 4,035,718. The tool has at least one multi-turn coil wound on a core of non-magnetic material. The coil is coupled to the electronic circuitry of the tool and cooperatively arranged for periodically applying a strong DC polarizing magnetic field, B.sub.P, to the formation in order to align proton spins approximately perpendicular to the earth's field, B.sub.E. The characteristic time constant for the exponential buildup of this spin polarization is the spin-lattice relaxation time, T.sub.1. At the end of polarization, the field is rapidly terminated. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to B.sub.E and therefore precess about the earth's field at the Larmor frequency f.sub.L =.gamma.B.sub.E. The Larmor frequency in the earth's field varies from approximately 1300 to 2600 Hz, depending on location. The spin precession induces in the coil a sinusoidal signal of frequency f.sub.L whose amplitude is proportional to the number of protons present in the formation. The tool determines the volume of free fluid in the formation. Additives in the borehole fluid are required to prevent the borehole fluid signal from dominating the formation signal. Also, there is necessarily a significant wait time before transients die down so that the coil can be used for detecting relatively small magnetic resonance signals.
A further nuclear magnetic resonance approach employs a locally generated static magnetic field, B.sub.o, which may be produced by one or more permanent magnets, and RF antennas to excite and detect nuclear magnetic resonance (using, for example, the type of RF pulse sequence first described above), to determine porosity, free fluid ratio, and permeability of a formation. See, for example, U.S. Pat. Nos. 4,717,878 and 5,055,787.
As pointed out in the referenced copending Application, the tools and techniques developed in the prior art have various drawbacks that limit their utility in practical applications. These limitations include, among others, one or more of the following: a shallow depth of investigation, difficulty in obtaining interpretable results in washed out formations or where certain types of invasion have occurred, restrictions on the shape and size of the region of investigation, the need for treating of the borehole fluid, and the need for significant waiting between transmission and receiving.
It is among the objects of the present invention to address limitations of prior art nuclear magnetic resonance logging techniques and apparatus, and to devise improved logging methods and equipment for obtaining magnetic resonance characteristics of earth formations surrounding a borehole.